The present invention relates to the field of photography and, more particularly, to a cold cathode discharge light source for use in photographic enlargers or printers.
The production of color photographs involves the use of a source of light. The source of light has to be adjusted to vary the color balance. One method for variation of the color balance is to use a "white" light source and vary balance by the use of subtractive yellow, cyan and magenta filters between the light source and the negative. An alternate method for color balancing a light source is the additive system wherein individual sources of red, green and blue light are combined or additively mixed to produce "white" light. The advantage of this alternative method is that the color of the composite light may be continuously varied over a wide range by varying amounts of the three primary colors being used to illuminate the printing plane. For example, the intensity of each of the three sources may be adjusted to vary the composite color.
A major problem associated with the prior art additive light systems is the lack of efficiency in terms of light utilization. Generally, the inefficiency occurs at the point where the three primary colors are mixed or combined.
Some mixing systems use a frosted glass plate as a diffuser coupled with other reflective devices to mix the red, green and blue light. Other systems employ an integrating sphere or dome for combining the light by multiple internal reflections. U.S. Pat. No. 3,120,721 uses a "light integrating bar" for combining the three primary colors. U.S. Pat. No. 3,825,336 uses a variable color light source featuring a fibre optic light mixing device for receiving different colored light. U.S. Pat. No. 4,124,292 uses a plurality of different individual lamps associated with mirrors which selectively reflect the lamp with which it is associated, the reflected light being then filtered and passed into a color-mixing chamber to provide a light of the predetermined color composition. The mixing chamber concept is also presented in U.S. Pat. No. 4,188,111. U.S. Pat. No. 4,111,548 involves a rotating reflecting device followed by mixing of the beams. U.S. Pat. No. 3,345,509 discloses a radial arrangement of the light sources around a central axis and focused at a central point, there being at least two source of light for each primary color. U.S. Pat. No. 4,640,609 arranges the red light source and the blue and green light sources in parallel or opposed directions.
Three cold cathode tubes have been used as grids in layers, one on top of the other, of green and blue light for black and white enlargers. The latter is not suitable as a light source for a color enlarger. The inventor has tested a five layer arrangement of red, two blue and two green grids, but a practical device did not result.
Because of the relatively low efficiency of mixing devices as light transmitters, in order to provide adequate light levels at the negative for short exposure times, high wattage lamps must be employed in the mixing devices. This in turn causes excessive heat to be generated within the lighting system. The heating problem either limits the size of the lamps that can be used, thereby extending exposure time, or requires that complex heat reducing or cooling aids be built into the system.